Optical filtering is an important concept in optics, and is commonly involved in a variety of optical components and instruments. One example is to use optical filters for optical detectors. Optical detectors are normally sensitive to a broad spectrum of light so that light of broad range of lights all might be detected. Therefore it would be much more useful to have a material or a device that operates exactly in a reverse manner that it selectively transmits light only in a narrow range of frequencies within a broad spectrum.
A Fabry-Perot resonantor can be used to filter or demultiplex the light. Though the Fabry-Perot can extract a narrow band of wavelength, a transmission spectrum of Fabry-Perot cavity structure usually shows multiple peaks with narrow passband width. For most applications, this multiple peaks should be selected by using external bandpass filters.